Hypoxic interaction between heterologous promoters and enhancers. (A and C) P19 cells; (B and D) Hep3B cells. Transient expression of reporter constructs in 21% and 2% oxygen was assayed. Hypoxic inducibility (the ratio of Luc activity in 2% oxygen v21% oxygen) is indicated. pLEn(M4)LPLuc shows little hypoxic inducibility in both P19 and Hep3B cells (see also Fig 7). Each value is the mean ± SD of triplicate experiments.